During the perinatal period, nutrition (i.e., composition of food) undergoes substantial changes (placental nutrition; milk, solid food). During this period, functions of the gastrointestinal tract exhibit corresponding changes; the mechanisms responsible for these changes are not yet understood. To contribute to elucidation of these problems, in the proposed experiments we will explore the following closely related areas: a) To characterize the reactivity to hormones, especially thyroid hormones and EGF, and adaptability to variation in carbohydrate intake of the microvillar disaccharidases during various periods of life, i.e., fetal, suckling, weaning, and adult life, and consequently in aged animals. b) To evaluate the effect of early perinatal manipulations (precocious maturation) induced by hormones or by a sudden change in diet such as occurring during the premature weaning on the adaptability of carbohydrases in adult and aged rats. c) Since hormones are present in milk and their physiological role in most cases is not yet understood, to evaluate the handling, i.e., digestion, retention and absorption of hormones present in milk by the gastrointestinal tract of the sucklings. The hormones that we have chosen to study are: ACTH, TSH, T3 and EGF. The need for these studies is further stressed by the fact that neonates fed no breast milk lose access to these and other hormones. The questions to be answered are: a) does the handling of hormones by the gastrointestinal tract change postnatally; or b) has the oral delivery of these hormones a functional significance.